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PostPosted: Thu Jun 28, 2018 12:36 am 
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Location: Gloucestershire, UK
Please direct me to the thread if this is already answered... a very basic beginner's question I think.

Really it boils down to: What's the best way to construct a 2 leaf system with the outer leaf made of brick?
I don't want the whole wall to be more than 250mm thick if possible.

I am thinking of constructing my studio like this:
outer leaf laid from brick, then render & seal it.
inner leaf 2 layers of heavy drywall with green glue.
fill the gap with RW3 rock wool.

it's the last part that confuses me, how do I fill the gap with rockwool?
if I use a stud wall in between won't that couple the two leaves together?

So do I make two stud walls, one attached to the brick wall and one to hold up the drywall, and leave a small gap between them?
if that's right, perhaps I can just make the two stud walls slightly narrower than the thickness of rockwool so that the entire gap is filled with rockwool but the stud walls don't touch?

If I'm on the right track, what's the best way to attach the stud wall to the brick wall?

And I guess the inner stud wall needs to be mounted on some rubber?
Do I just screw the drywall to the stud and then seal all the screws?

Or am I completely confused?


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PostPosted: Thu Jun 28, 2018 12:50 am 
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What's the best way to construct a 2 leaf system with the outer leaf made of brick?
First, do the calculations to ensure that it really will isolated to the level you need, at the frequencies where you need it! If you don't calculate, then you are guessing. And guessing is never a good way to build a studio.

Quote:
I am thinking of constructing my studio like this:
outer leaf laid from brick, then render & seal it.
inner leaf 2 layers of heavy drywall with green glue.
fill the gap with RW3 rock wool.
Sounds about right, but you'll only know if you do the math! :)

Quote:
how do I fill the gap with rockwool?
That depends on how big your gap needs to be (between the brick surface and the inner-leaf frame). And to determine how big that gap needs to be, you have to do the math! :)

Quote:
if I use a stud wall in between won't that couple the two leaves together?
Yes it would, which is why you cannot do that! There are several methods for keeping the insulation in the gap, but the method that YOU will need depends on how big your gap is, and that depends on you doing the math...

Quote:
So do I make two stud walls, one attached to the brick wall and one to hold up the drywall, and leave a small gap between them?
No. You make ONE stud frame, separated from the brick wall by a gap whose size you will determine by doing the math (!), then based on that you determine which method will work for you to keep the insulation in place...

Quote:
If I'm on the right track, what's the best way to attach the stud wall to the brick wall?
Answer: don't! That would be a total waste of wood, time and money.


Quote:
And I guess the inner stud wall needs to be mounted on some rubber?
No, it doesn't. No need at all for that.

Quote:
Do I just screw the drywall to the stud and then seal all the screws?
You can either screw the drywall in place, or you can nail it in place. For the first method, you will need proper drywall screws, for the second method you will need proper drywall nails. In both cases, the nails or screws for the SECOND layer must be LONGER than the standard nails/screws you used on the first layer, because those second layer fasteners need to go through the extra thickness of the first layer, and the still penetrate the correct distance into the studs.

Quote:
Or am I completely confused?
If you don't know how to hang drywall, then you should consider either taking a short course on how to do that, or hire someone to do it for you. It is NOT just simply slapping up a sheet of drywall against the studs then hammering in a few nails here and there... There are techniques and guidelines, especially for studio walls (which need to be sealed air-tight, unlike normal house walls), and you also need to know how to cut the drywall to the right size, how to mud-and-tape, and how to fix any damage that you might cause....


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PostPosted: Thu Jun 28, 2018 2:41 am 
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Thanks so much. I will definitely be hiring people, I just want to be directing them to build it the right way!

Now to search for the info on how to do the calculations I need to do.......


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PostPosted: Thu Jun 28, 2018 2:49 am 
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:thu:


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PostPosted: Thu Jun 28, 2018 2:56 am 
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OK please help me do the calculation!
Or tell me where to look.... my google skills are letting me down.
Perhaps I need to buy A BOOK???


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PostPosted: Thu Jun 28, 2018 2:59 am 
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The equations for calculating total isolation of a two-leaf wall are simple:

First, for a single-leaf barrier you need the Mass Law equation:

TL = 14.5 log (M * 0.205) + 23 dB

Where: M = Surface density in kg/m2

For a two-leaf wall, you need to calculate the above for EACH leaf separately (call the results "R1" and "R2").

Then you need to know the resonant frequency of the system, using the MSM resonance equation:

f0 = C [ (m1 + m2) / (m1 x m2 x d)]^0.5

Where:
C=constant (60 if the cavity is empty, 43 if you fill it with suitable insulation)
m1=mass of first leaf (kg/m^2)
m2 mass of second leaf (kg/m^2)
d=depth of cavity (m)

Then you use the following three equations to determine the isolation that your wall will provide for each of the three frequency ranges:

R = 20log(f (m1 + m2)) - 47 ...[for the region where f < f0]
R = R1 + R2 + 20log(f x d) - 29 ...[for the region where f0 < f < f1]
R = R1 + R2 + 6 ...[for the region where f > f1]

Where:
f0 is the resonant frequency from the MSM resonant equation,
f1 is 55/d Hz
R1 and R2 are the transmission loss numbers you calculated first, using the mass law equation

And that's it! Nothing complex. Any high school student can do that. It's just simple addition, subtraction, multiplication, division, square roots, and logarithms.

:)


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PostPosted: Thu Jun 28, 2018 6:43 am 
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Ah that was very very informative, thank you so much.
If I followed the instructions correctly, I worked out that for a leaf of brick / filled air gap / leaf of drywall construction:

A 100mm air gap gives me about -6dB better performance than a 50mm gap for most of the range but far better (approx 40dB!) around the 35-50Hz zone where my bass drums are likely to be.

The difference between using 2 x 12.5mm acoustic drywall vs 2 x 15mm was negligible, but going down to a single sheet of drywall was about as bad as halving the gap from 100mm to 50mm.

100mm air gap with two sheets of 12.5mm acoustic drywall seems to have the potential for -60dB around 32Hz, rising to around -83dB above 550Hz.
Obviously it won't be this good in the real world with doors, air vents, roof etc... but it seems very acceptable.

My wall would be about 240mm thick which is also acceptable.

150mm air gap takes the wall thickness up to 290mm and gives another -3dB or so which is not that impressive but makes a big difference down around 25-30Hz. I would imagine my small speakers would barely produce sound that low anyway and the loss of space is too much. I am very constrained in terms of the width of the area I can build on.

Does this all seem like it might be about right to you?


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PostPosted: Thu Jun 28, 2018 8:20 am 
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Here's an excel file I put together so you can quickly type in variables and it will show you your isolation values at different frequencies.

Hopefully it speeds up your calculation process:

http://www.mediafire.com/file/shl5c966q ... .xlsx/file

Greg

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PostPosted: Thu Jun 28, 2018 5:37 pm 
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Thanks.

By the way, I've included the width of the stud wall as part of my air gap, is that correct?


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PostPosted: Fri Jun 29, 2018 7:09 am 
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Quote:
By the way, I've included the width of the stud wall as part of my air gap, is that correct?
The "air gap" in the equations is the distance across the full cavity inside your wall, between the leaves. Distance from the face of the brick, that faces the cavity, up to the face of the drywall that faces the cavity. That's the "depth" of the cavity, and that's what matters for the MSM calculations.

Your conclusions in your previous post are very correct (6 dB difference for halving the air gap or halving the mass on one leaf, etc.), but your overall isolation numbers seem a bit high: "100mm air gap with two sheets of 12.5mm acoustic drywall seems to have the potential for -60dB around 32Hz, rising to around -83dB above 550Hz.". What density are you using for the brick wall, and for the drywall?

Quote:
150mm air gap takes the wall thickness up to 290mm and gives another -3dB or so which is not that impressive but makes a big difference down around 25-30Hz.
Yep!

Quote:
I would imagine my small speakers would barely produce sound that low anyway
Don't "imagine"! Don't "guess"! Find out for sure. Check the manuals or on-line specs for your speakers, to find out how much energy it is putting out down that low. And check that against your music needs! For example, if you track and mix a lot of 6-string bass, and don't have good isolation below 50 Hz, then you have a problem. Ditto if you mix 6-string bass and your speakers don't put out much sound below 50 Hz...

Always match your speakers to the task at hand, and to the room, then match the isolation to that.

- Stuart -

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PostPosted: Fri Jun 29, 2018 6:41 pm 
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Bricks seem to be about 1800kg/m3.
They are 92mm thick.
I think that gives me about 160 kg/m2.

I was looking at Gyproc acoustic plasterboard which is quoted as being 10.6 kg/m2, so I presume if I am using two layers I can just double the figure to 21.2 kg/m2.

TL of brick = 45
TL of 2 layers of Gyproc = 32.25

For a 10mm air gap, f0 = 31Hz
f1 = 550Hz


My next question is:
the rockwool will be inside the gaps in a studwall to which the drywall is screwed. If the rockwool is thicker than the stud, so the stud is away from the brick wall, does it matter if the rockwool touches the brick? ie. Should I fill the ENTIRE gap or leave a small gap of actual air?


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PostPosted: Sat Jun 30, 2018 2:06 am 
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Quote:
100mm air gap with two sheets of 12.5mm acoustic drywall seems to have the potential for -60dB around 32Hz, rising to around -83dB above 550Hz.

I'm not seeing that. Here's the rough calculations:

f0 = C [ (m1 + m2) / (m1 x m2 x d)]^0.5
= 43 ((160+21) / (160*21*0.1)) ^0.5
= 43 (181 / (336)) ^0.5
= 43 (0.54) ^0.5
= 43 x 30.74
= 32 Hz

R = 20log(f (m1 + m2)) - 47
= 20log(32 (160 + 21)) - 47
= 20log(32 (181)) - 47
= 20log(5792) - 47
= 20 (3.76) - 47
= 75 - 47
= 28 dB

So your isolation at resonance is about 28 dB, and certainly not 60 dB. I'm not sure how you got 60 dB there.

- Stuart -

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PostPosted: Wed Jul 04, 2018 12:30 am 
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Right. I got the same results as you from the calculation.
The thing that the formulae you gave doesn't tell me is:
What is happening in the transitions between the frequency ranges covered by the 3 equations?
ie. just above the resonant frequency.

For example at 40 Hz, R is given as 30dB by the first equation, and 60dB by the second equation.
F0 is about 31 Hz so therefore 40>f0 and we use the second of the three equations to find R=60, but I suspect there is something more subtle going on near the resonances.
I wonder what the real R is for 40Hz, or 35Hz or 32Hz?

On the positive side we are going to survey the site tomorrow and figure out how much space there is and this will tell me exactly how thick the walls can be. If i had infinite space I would make the air gap a metre thick but it's a narrow plot unfortunately!


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PostPosted: Thu Jul 05, 2018 9:00 am 
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I've been running the calculation using the spreadsheet ( so useful - thanks!) on various possibilities in my place. I was planning on floating an inner stud wall in a 30' x 20' brick room.

I have a mix of solid brick and cavity walls in my building (an Edwardian edifice - built in 1910 with many alterations over the years) My bricks have a surface density of around 225Kg/m2 per wythe and I will probably use a sheet material of around 15Kg/m2 for the second leaf.

With a 4.5" gap and insulation in the gap, the resonant frequency of the wall system is around 34hz. Below this frequency the TL drops by half from around 61 db to around 31db. This does seem extreme, and it would seem that only the kick drum range would be an issue..

This all seems very adequate for my purposes, but I suspect other issues will have a bigger effect than the wall gap. How can one ascertain the point where increasing the air gap or 2nd leaf density will be compromised by other considerations?

How does one account for flanking transmission when looking at the design of such a wall?

In my case I also have windows that are 8' x 4'. I suspect those will have a bigger effect on my max TL in the room than the wall structure.

As an aside, I wonder whether floating the inner stud wall is even worthwhile - I could put studding directly onto the brick much more easily - and the cost of floating the wall versus the extra TL is hard to calculate in this case.

Any thoughts?


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PostPosted: Thu Jul 05, 2018 10:17 am 
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Welcome Leland2 (whatever your real name is)!

With as much detail as you provided, I'd suggest making your own thread on the design forum. From there, keeping all of your questions to one thread will allow everyone to follow all of your build details from the beginning to the end.

Quote:
I've been running the calculation using the spreadsheet ( so useful - thanks!)

I'm glad my spreadsheet is making your life easier!

Quote:
I was planning on floating an inner stud wall in a 30' x 20' brick room.

What benefit do you think floating your walls will provide? In order to properly do this, you'd need to know the exact weight of your walls (and everything mounted to them such as acoustic treatment) as well as the ceiling. Then you'd have to use the correct material with the correct deflection to isolate your walls at as low of frequency as possible, all while maintaining structural integrity. If your isolation level requires to go to the lengths of floating walls, you're best bet is to float your whole inner leaf room, floor and all. That = insane money and design. If you're on ground level, Earth is the best to build on and you won't need to float anything except your speakers when soffit mounting.

Quote:
I will probably use a sheet material of around 15Kg/m2 for the second leaf.

What material is this?

Quote:
but I suspect other issues will have a bigger effect than the wall gap.

Mass and the gap (filled with appropriate insulation) are the two factors we can adjust. The only other factor after those would be attention to detail during the build. Don't skimp anywhere. Make sure the mass is maintained EVERYWHERE -- including windows, doors, electrical penetrations and HVAC silencer boxes!

Quote:
How can one ascertain the point where increasing the air gap or 2nd leaf density will be compromised by other considerations?

Point in which factor? The spreadsheet will allow you to easily type in smaller or larger gaps and see if adding that extra half an inch would be worth it. There is a point where adding extra gap yields very little transmission loss increase. So, balance out leaf mass and gap. Some people have huge spaces that are able and willing to have 2 foot gaps. Most people are desperate to keep 1/2". Where we are desperate for space, we often have to spend more money on things like MLV or a ton of Green Glue compound. And regarding becoming compromised, that would only happen with attention to detail during the build as mentioned above.

Quote:
How does one account for flanking transmission when looking at the design of such a wall?

Again, flanking would almost certainly occur when trying to float your wall and ceiling assembly. You can (for a ton of money) avoid flanking by floating your entire inner room on a very heavy concrete slab. The best solution is to just anchor your walls to your floor.

Quote:
In my case I also have windows that are 8' x 4'. I suspect those will have a bigger effect on my max TL in the room than the wall structure.

Your windows can be as large as you want. Just use proper windows. Each leaf must maintain the same surface density as your wall. Typically if your window is a 1/3rd the thickness as your drywall, you're good to go in terms of mass. Make sure you use laminated glass. Preferably built with acoustic PVB, not regular PVB (this is a luxury and will increase performance, but regular PVB works well too).

Quote:
I wonder whether floating the inner stud wall is even worthwhile.

As I've written above, it is not worth floating unless you need upwards of 100dB of isolation.

Quote:
I could put studding directly onto the brick much more easily

No. You don't want to anchor your studs to your brick outer leaf! Leave the gap and make sure your ceiling for your inner leaf is sitting on your inner leaf walls and not touching the outer leaf anywhere!

Quote:
and the cost of floating the wall versus the extra TL is hard to calculate in this case

Probably not. Tens or hundreds of thousands (depending on your room size) probably isn't worth it.

Again, I'll remind you to start a design thread where we can follow your design come to fruition!

Greg

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